Organic electroluminescence device

ABSTRACT

The present invention provides an organic EL device, by which an anode and cathode are prevented from being short-circuited in case of an emitting cell is blown out. The present invention includes an anode on a substrate, an organic layer on the anode, a cathode on the organic layer, and a short-prevention layer having a tensile stress on the cathode.

This application claims the benefit of the Korean Application No.10-2003-0086832 filed on Dec. 2, 2003, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic electroluminescence device,by which an anode and cathode are prevented from being short-circuitedin case of an emitting cell is blown out.

2. Discussion of the Related Art

Generally, an organic electroluminescence (hereinafter abbreviated EL)device using electroluminescence of an organic material has a structurethat an organic hole transport layer or organic luminescent layer isinserted between upper and lower electrodes.

Many efforts are made to research and development for the organic ELdevice capable of high brightness luminescence by a DC drive of lowvoltage.

Specifically, a bottom emission type organic EL device, as shown in FIG.1, consists of an anode 12 on a substrate 11, an organic layer 13 on theanode 12, and a cathode 14 on the organic layer 13.

The organic layer 13 is configured with at least one layer formed of anorganic material.

For example, the organic layer 13 consists of a hole injection layer(HIL) 13 a, a hole transport layer (HTL) 13 b, an emitting layer 13 c,and an electron transport layer 13 d.

The cathode 14 is operative in injecting electrons and reflecting anemitted light. The anode 12 is formed of a transparent material such asITO (indium tin oxide).

In the above-configured organic EL device, if the anode 12 and cathode14 are connected to positive and negative terminals of a DC power,respectively, holes injected from the hole injection layer 13 a move tothe emitting layer 13 c via the hole transport layer 13 b.

Meanwhile, electrons injected from the cathode 14 move to the emittinglayer 13 c via the electron transport layer 13 d.

Hence, the electrons and holes having moved to the emitting layer 13 care coupled to each other to emit light.

The light, as indicated by an arrow in the drawing), emitted from theemitting layer 13 c is directly discharged outside via the cathode 12.And, the light emitted in a direction of the cathode is reflected on thecathode 14 to be discharged outside via the anode 12.

Thus, the cathode 14 of the organic EL device is operative in injectingelectrons, reflecting light, and reducing resistance.

Yet, in case that an emitting cell, as shown in FIG. 2, is blown out byinternal and/or external stresses, the cathode 14 is bent toward theorganic layer 13.

Moreover, heat generated from the explosion of the emitting cell maymelt the cathode 14 down in a direction of the anode 12. Hence, it ishighly probable that the bent cathode 14 can be short-circuited with theanode 12.

The organic devices are generally categorized into active matrix devicesand passive matrix devices.

However, since the short circuit occurrence between the cathode 14 andthe anode 12 causes a dead pixel in the active matrix device, ashort-occurring pixel becomes unusable.

Besides, since the short circuit occurrence causes line failure in thepassive matrix device, a line including the shorted pixel becomesunusable.

Therefore, the shorted cathode and anode lowers reliability of theorganic EL device.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an organicelectroluminescence (EL) device that substantially obviates one or moreproblems due to limitations and disadvantages of the related art.

An object of the present invention is to provide an organic EL device,by which an anode and cathode are prevented from being short-circuitedin case of an emitting cell is blown out.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, anorganic EL device according to the present invention includes an anodeon a substrate, an organic layer on the anode, a cathode on the organiclayer, and a short-prevention layer having a tensile stress on thecathode.

Preferably, the short-prevention layer is formed of metal having highthermal conductivity.

Preferably, the short-prevention layer is formed of either one selectedfrom the group consisting of Cr, Mo, Ni, Ti, Co, Ag, Au, Al, Pt, and Pdor an alloy of at least two selected from the group.

Preferably, the short-prevention layer is formed of a material having amelting point higher than that of the cathode.

Preferably, the short-prevention layer is formed by resistance heating.

Preferably, the short-prevention layer is formed by either E-beamprocessing.

Preferably, the short-prevention layer is formed by sputtering.

Preferably, the short-prevention layer is formed at a temperature withina range leaving the organic layer intact.

More preferably, the temperature is equal to or lower than 80° C.

More preferably, the short-prevention layer is formed by the sputteringat a pressure of 0.5˜10 mTorr, a power density of 0.1˜8 W/cm², and asubstrate bias of (−)500˜0V.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a cross-sectional diagram of an organic EL device according toa related art;

FIG. 2 is a cross-sectional diagram of a cathode of an organic EL deviceaccording to a related art in case of having a blown-out emitting cell;

FIG. 3 is a cross-sectional diagram of an organic EL device according tothe present invention;

FIG. 4 is a cross-sectional diagram of a cathode of an organic EL deviceaccording to the present invention in case of having a blown-outemitting cell; and

FIG. 5A and FIG. 5B are graphs of tensile stress variations of asubstrate according to power and pressure in depositing a Cr layer atthe room temperature.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

FIG. 3 is a cross-sectional diagram of an organic EL device according tothe present invention.

Referring to FIG. 3, an organic EL device according to the presentinvention includes an anode 22 on a substrate 21, an organic layer 23 onthe anode 22, a cathode 24 on the organic layer 23, and ashort-prevention layer 25 having a tensile stress on the cathode 24.

The organic layer 23 is configured with at least one layer formed of anorganic material.

For instance, the organic layer 23 includes a hole injection layer (HIL)23 a, a hole transport layer (HTL) 23 b, an emitting layer 23 c, and anelectron transport layer (ETL) 23 d, which are sequentially stacked onthe anode 22.

The cathode 24 is operative in injecting electrons, reflecting lightemitted from the emitting layer 23 c in a direction of the substrate 21,and reducing resistance.

In doing so, the tensile stress is additionally provided to the stackedlayer configured with the cathode 24 and the short-prevention layer 25as well as the foresaid roles of the related art cathode. Hence, it canbe said that the stacked layer configured with the cathode 24 and theshort-prevention layer 25 is an extended cathode.

And, the short-prevention layer 25 is formed of one of Cr, Mo, Ni, Ti,Co, Ag, Au, Al, Pt, Pd, and the like or an alloy consisting of at leasttwo selected from the group consisting of Cr, Mo, Ni, Ti, Co, Ag, Au,Al, Pt, Pd, etc.

Moreover, the short-prevention layer 25 is formed by resistance heating,E-beam processing, or sputtering.

In case of forming the short-prevention layer 25 by sputtering, theranges of pressure, power density, substrate bias, and temperatureshould be appropriately set up not to cause damage to the organic layer23.

As a result of test, the above sputtering is preferably carried out atthe pressure of 0.5˜10 mTorr, power density of 0.1˜8 W/cm², substratebias of (−)500˜0V, and temperature equal to or lower than 80° C.

Once the short-prevention layer 25 is formed on the cathode 24, thetensile stress of the short-prevention layer 25 affects the cathode 24underneath.

FIG. 4 is a cross-sectional diagram of a cathode of an organic EL deviceaccording to the present invention in case of having a blown-outemitting cell.

Referring to FIG. 4, in case that an emitting cell is blown out byinternal and/or external stresses, the cathode 24 is bent in a directionopposite to the organic layer 23 due to the tensile stress of theshort-prevention layer 25.

Hence, it is able to prevent the anode 22 and cathode 24 from beingshorted with each other.

Once the cathode 24 is bent in the direction opposite to the organiclayer 23 due to the tensile stress of the short-prevention layer 25, apredetermined gap is naturally provided over the organic layer 23.Hence, heat generated from the explosion of the emitting cell candissipate quickly via the gap.

As the internal heat is externally dissipated, the cathode 24 isprevented from being melted by the heat generated from the explosion ofthe emitting cell. Hence, the cathode 24 can avoid contacting with theanode 22 or causing damage to the organic layer 23.

Moreover, the tensile stress of the short-prevention layer 25 affectsthe cathode 24 to suppress hillock formation occurring in the cathode24.

Therefore, the problems of the dead pixel, dark spot, line failure, andthe like can be solved.

Meanwhile, the variation of the tensile stress according to a depositioncondition of the short-prevention layer 25 is taken into considerationas follows, in which the short-prevention layer 25 is formed of Cr forexample.

FIG. 5A and FIG. 5B are graphs of tensile stress variations of asubstrate according to power and pressure in depositing a Cr layer atthe room temperature, for which DC magnetron sputter equipments areused.

Namely, a Cr layer is deposited about 1,500 Å thick on a siliconsubstrate suing DC magnetron sputter equipments. And, curvatures of thesubstrate are measured prior to and after the Cr layer deposition tocalculate a stress.

Referring to FIG. 5A, a tensile stress is 1.3 Gpa at a process power of200 W and Ar ambience of 0.6 mTorr. Yet, the tensile stress is equal toor smaller than 1 Gpa at the process power of 600 w.

Hence, it can be confirmed that the tensile stress is inverseproportional to the process power.

Referring to FIG. 5B, the tensile stress temporarily increases but thendecreases according to the pressure increment to 10 mTorr from 0.6 mtorrat the room temperature with a fixed power of 640 W.

In depositing a layer, a compression stress is generally converted to atensile stress in case of raising a pressure or in case of lowering adeposition power.

Such a general principle corresponds to the result of the test data inFIG. 5A and FIG. 5B in aspect of the power variation only, but fails tocorrespond to the same result in aspect of the pressure variation.

The inconsistency between the test result and the general principle isattributed to the intrinsic property of Cr used as the short-preventionlayer. By the way, the test result, which has similarity to the resultdisclosed in A. Misra et al., is reliable.

By considering the Cr layer has a wide window having the tensile stress,an organic EL device is substantially fabricated using the Cr layer asthe short-prevention layer.

As a result of fabricating the organic El device having theshort-prevention layer provided thereto, the number of dark spots of theorganic EL device of the present invention is reduced to a half of thatof the related art. And, an initial line failure occurrence time of thepresent invention is enhanced from 520 hours to 1,000 hours, which istwice longer than that of the related art.

The test result explains the aforesaid usefulness of theshort-prevention layer. Namely, in case that the emitting cell ispartially blown out, the cathode 24 becomes simply cut open not to beeasily shorted with the anode 22 and the internal heat is efficientlydissipated outside.

Moreover, the short-prevention layer suppresses the formation ofhillock.

Besides, adhesiveness between the cathode 24 and the anode 23 isenhanced by the physical energy of Cr atom on depositing theshort-prevention layer.

Meanwhile, any material, which has a melting point higher than that ofthe cathode 24 and enables the short-prevention layer 25 to have thetensile stress under the aforesaid conditions, can be used as a rawmaterial of the short-prevention layer 25.

Preferably, the short-prevention layer 25 is formed of metal having highheat conductivity with facilitation of its deposition.

Besides, a material having a high melting point is effective to sustaina shape instead of being melted in case of the explosion of the emittingcell.

Accordingly, the present invention provides the following effects oradvantages.

First of all, in case of the explosion of the emitting cell, the cathodeis bent in the direction opposite to the anode due to the tensile stressof the short-prevention layer. Hence, the probability of the shortoccurrence between the cathode and the anode is lowered and thegenerated heat can be dissipated via the gap provided by the bentcathode.

Therefore, the problems of the dead pixel, dark spot, line failure, andthe like can be solved.

Secondly, the short-prevention layer having the tensile stress canreduce the hillock occurrence in the cathode.

Thirdly, since the short-prevention layer is formed of the materialhaving the melting point higher than that of the cathode, the cathode isless melted in case of the explosion of the emitting cell to lower theprobability of the short occurrence between the cathode and the anode.

Finally, adhesiveness between the cathode and the anode is enhanced bythe physical energy of Cr atom on depositing the short-prevention layer,whereby a device lifetime is elongated.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. An organic EL device comprising: an anode on a substrate; an organiclayer on the anode; a cathode on the organic layer; and ashort-prevention layer having a tensile stress on the cathode.
 2. Theorganic EL device of claim 1, wherein the short-prevention layer isformed of metal having high thermal conductivity.
 3. The organic ELdevice of claim 1, wherein the short-prevention layer is formed ofeither one selected from the group consisting of Cr, Mo, Ni, Ti, Co, Ag,Au, Al, Pt, and Pd or an alloy of at least two selected from the group.4. The organic EL device of claim 1, wherein the short-prevention layeris formed of a material having a melting point higher than that of thecathode.
 5. The organic EL device of claim 1, wherein theshort-prevention layer is formed by resistance heating.
 6. The organicEL device of claim 1, wherein the short-prevention layer is formed byeither E-beam processing.
 7. The organic EL device of claim 1, whereinthe short-prevention layer is formed by sputtering.
 8. The organic ELdevice of claim 1, wherein the short-prevention layer is formed at atemperature within a range leaving the organic layer intact.
 9. Theorganic EL device of claim 8, wherein the temperature is equal to orlower than 80° C.
 10. The organic EL device of claim 7, wherein theshort-prevention layer is formed by the sputtering at a pressure of0.510 mTorr, a power density of 0.1˜8 W/cm², and a substrate bias of(−)500˜0V.